1. Field of the Invention
The present invention relates to measures for improving the reliability of microwave sensors (hereinafter, referred to as “MW sensor”).
2. Description of the Related Art
MW sensors (first conventional MW sensors) that transmit microwaves to a detection area and detect an intruder by receiving reflected waves (microwaves modulated due to the Doppler effect) from the intruder when the intruder is present in the detection area have been known as a conventional crime prevention device (e.g., JP7-37176).
Furthermore, sensors that measure the distance to an object such as an intruder by utilizing a plurality of microwaves having different frequencies are also known as one type of MW sensors.
This type of MW sensors (second conventional MW sensors) transmit, for example, two kinds of microwaves having different frequencies to a detection area and detect a phase difference of the two IF (infrared, e.g. microwave) signals based on the respective reflected waves. This phase difference correlates with the distance to an object, and the larger the distance to an object is, the larger the phase difference is. In other words, the distance to an object can be measured by obtaining this phase difference.
Furthermore, it is possible to determine whether or not an object is moving in a detection area by identifying a temporal change of the phase difference. Thus, for example, only the object that is moving can be determined as an object to be detected. Hereinafter, the operation of detecting the phase difference of IF signals in this type of MW sensors (third conventional MW sensors) will be described.
In the case where the IF signals based on reflected waves of two microwaves having different frequencies have sine waves IFout 1 and IFout 2 (having a phase difference corresponding to the distance to an object) as shown in FIG. 8A, rectangular waves A and B that are shaped from these IF signals are such as those shown in FIG. 8B, respectively. Then, the distance to an object can be measured by detecting the phase difference between the rectangular waves A and B (phase difference Δt in the rising portions of the rectangular waves in FIG. 8B). Furthermore, it is possible to identify a movement of an object in a detection area (whether or not an object is approaching or going away from the MW sensor) by identifying a temporal change of the phase difference of these rectangular waves A and B.
However, there is the following problem when the third conventional MW sensor is used as a crime prevention sensor to identify a temporal change of the phase differences and determine only an object moving in a detection area as an object to be detected.
When the third conventional MW sensor is provided outdoors, the phase difference between the rectangular waves A and B can be generated by, for example, wavering of plants or the like by breeze, and thus it is possible to detect erroneously the plants or the like as an object to be detected (intruder). Similarly, when the third conventional MW sensor is provided indoors, the phase difference between the rectangular waves A and B (see FIG. 8) also can be generated by wavering of blinds or curtains due to a rotating operation of a fan for ventilation or breeze, so that also in this case, it is possible to detect erroneously objects other than an intruder as an object to be detected (intruder).
In order to solve the above problem, the present invention has an object of providing a MW sensor that determines correctly whether or not an object moving in the detection area is an object to be detected and eliminates detection errors.